Field of the Invention
This invention relates generally to a method for acoustic depth discrimination in shallow water having a thermocline, and more particularly to a method for determining whether a distant sound source is near the surface or submerged in the water using a single hydrophone or a horizontal hydrophone array.
Description of the Related Art
Current approaches for depth classification are based on Matched Field Processing (“MFP”) and its extensions to more robust methods, such as acoustic mode scintillation and acoustic mode filtering. (See, e.g., A. B. Baggeroer, W. A. Kuperman, Henrik Schmidt, “Matched field processing, Source localization in correlated noise as an optimum parameter estimation problem,” J. Acoust. Soc. Am. 83(2), 571-587, 1988; V. E. Premus, “Modal scintillation index: A physics based statistic for acoustic source discrimination,” J. Acoust. Soc. Am. 105(4), 2170-2180, 1999; and V. E. Premus, J. Ward, and C. D. Richmond, “Mode filtering approaches to acoustic source depth discrimination,” IEEE Underwater Acoustic Signal Processing Workshop. 1415-1420, 2004, all three articles being incorporated herein by reference). The MFP algorithms require precise knowledge of the propagation environmental parameters, including the geoacoustic properties (e.g. sediment sound-speed, density, and attenuation). In practice, range and depth estimation by MFP-based methods becomes problematic due to insufficient knowledge of the environmental parameters. Mode-scintillation and mode-filtering methods seek to reduce the acoustic source localization problem to a two-class discrimination problem. The mode-scintillation method exploits the high or low Scintillation Index (“SI”) property of acoustic modes generated by a near-surface or a submerged source, respectively. Similarly, the mode-filtering method seeks to discriminate low or high order acoustic modes generated by a near-surface or a submerged source, respectively. Both methods require either densely populated vertical hydrophone arrays, or horizontal arrays that are carefully placed in the water column so as to be away from ocean boundaries. Neither of these requirements is practical nor considered for existing surveillance systems. In addition, both methods require water column sound-speed profile information, as well as the critical angle of the seabed. Development of a more robust depth-discrimination method that requires minimum environmental information and uses acoustic data from existing surveillance systems is highly desirable; these are systems such as horizontal hydrophone arrays or single-hydrophone sensors of distributed networks.